Page:Popular Science Monthly Volume 29.djvu/321

Rh dragon-heads, each of which holds a ball in its mouth. Underneath these heads there are eight frogs so placed that they appear to watch the dragon's face, so that they are ready to receive the ball if it should be dropped. All the arrangements which cause the pillar to knock the ball out of the dragon's mouth are well hidden in the bottle. When an earthquake occurs, and the bottle is shaken, the dragon instantly drops the ball and the frog which receives it vibrates vigorously. Any one watching this instrument can easily observe earthquakes. With this arrangement, although one dragon may drop a ball, it is not necessary for the other seven dragons to drop their balls unless the movement has been in all directions: thus we can easily tell the direction of an earthquake. Once upon a time a dragon dropped its ball without any earthquake being observed, and the people therefore thought the instrument of no use, but after two or three days a notice came, saying that an earthquake' had taken place at Ròsei. Hearing of this, those who doubted the use of this instrument began to believe in it again. After this ingenious instrument had been invented by Chôko, the Chinese Government wisely appointed a secretary to make observations on earthquakes." This, the most ancient of the whole class, is closely resembled by some of the instruments of modern times.

The Japanese have an instrument consisting of a magnet holding up a nail, which, when shaken off, starts the train of an alarum, but this does not seem to have ever acted with success. Other seismoscopes depend upon the overthrow of a round column of wood or metal, the projection of balls which are connected with electric circuits, or the disturbance of liquids. Some seismographs depend upon the motions of a pendulum, which may be made to show whether the direction of the shock has been constant or variable, and the maximum extent of its motion in various directions. Other instruments are formed by various adjustments of movable bodies, or with springs and adaptations of clock-work. For a complete seismograph we require three distinct sets of apparatus—an apparatus to record horizontal motion, one to record vertical motion, and one to record time. These principles are all embodied in the Gray and Milne seismograph, which is now in use in Japan. In this apparatus (Fig. 2) two mutually rectangular components of the horizontal motion of the earth are recorded on a sheet of smoked paper wound round a drum, D, kept continuously in. motion by clock-work, W, by means of two conical pendulum seismographs, C. The vertical motion is recorded on the same sheet of paper by means of a compensated-spring seismograph, S. L. M. B. The time of occurrence of an earthquake is determined by causing the circuit of two electro-magnets to be closed by the shaking. One of these magnets relieves a mechanism, forming part of a time-keeper, which causes the dial of the time-piece to come suddenly forward on the hands and then move back to its original position. The hands are.